Plastic closures have been commonly made by injection molding or compression molding In plastic closures made by compression molding as shown, for example, in U.S. Pat. Nos. 3,210,805, 4,434,754, and 4,418,828, incorporated herein by reference, it is also common to compression mold a sealing liner in place in the closure after the closure has been formed. Such sealing liners may comprise polyvinyl chloride plastic or thermoplastic elastomeric compositions. Polyvinyl chloride has a tendency to overheat during extrusion and further has inherent problems because of the removal torque when the sealing liner is used on a PET container. Thermoplastic polymeric compositions are more expensive, more difficult to produce in that the compounding necessary results in inconsistent quality and consistency. Furthermore, such compositions produce problems of torque removal on glass containers. U.S. Pat. Nos. 4,807,772 and 4,846,362 disclose a compression molded liner being made of a blend of polyethylene or polypropylene and thermoplastic elastomeric copolymer so that the liner is removable.
As a result of the problems with both types of compositions, it has been necessary for a user to maintain costly inventories of closures with both types of sealing liners in order to accommodate use with both plastic and glass containers.
In order to produce a satisfactory sealing liner that would be applicable to both plastic and glass containers, it must be necessary to not only formulate the composition but also consistently extrude and cut off charges of the composition for compression molding in apparatus such as shown, for example, in U.S. Pat. Nos. 3,955,605, 4,080,136 and 4,277,431. The closure must also match other closure requirements including satisfactory removal torques, carbonation retention, carbonation retention under top load and satisfactory functioning on a container when cycled through environmental changes of temperature and humidity.
Accordingly, among the objectives of the present invention are to provide a compression molded plastic closure having a compression molded sealing liner which will overcome the problems inherent in the prior sealing liners; which will operate satisfactorily on both plastic and glass containers; which is relatively lower in cost; and which can be readily processed and meet the requirements for a satisfactory closure.